Male Reproductive System
The male reproductive system is part of a system called the urogenital system. The urogenital system consists of the urinary system and the reproductive system, both of which in the male, merge (sharing some common ducts, e.g. the urethra). Thus it is important to distinguish between the urinary system and the reproductive system, because the urogenital system consists of the excretory organs (urinary organs) including the kidneys, ureters, urinary bladder, and the urethra, and the reproductive organs.
The urinary system functions primarily to:
- Remove waste product from the body (mainly urea and uric acid)
- Regulate electrolyte balance e.g. sodium, potassium and calcium
- Control blood volume and maintain blood pressure
In contrast, functions of the reproductive system (in the male), include:
- to produce, maintain, store and transport spermatozoa (the male reproductive cells) and the cells’ supportive fluid – the semen
- to discharge those reproductive cells within the female reproductive tract, during sexual intercourse
- to produce and secrete the male sex hormones responsible for maintaining the male reproductive system
Male Genital Organs
The male reproductive system is the system concerned with reproduction – production of gametes for fertilization of the female’s ovum (egg or gamete), and nourishment of the developing fetus. Like other body systems, the male reproductive system is composed of organs, but in addition, also comprising of some glands and ducts.
The organs of the male reproductive (or genital) system can be divided into two groups:
The internal genital organs include the:
- testes (singular: testis)
- epididymides (singular: epididymis)
- ductus deferentes (singular: ductus deferens)
- seminal glands (seminal vesicles)
- ejaculatory ducts
- bulbo-urethral glands (Cowper glands)
The seminal glands, prostate gland and the bulbourethral glands constitute the accessory glands.
The testes and epididymis are housed in the scrotum; however, they are considered internal genital organs on the basis of their developmental position and homology with the internal female ovaries.
The following is a very brief account of the development of the male reproductive system. However, the main events are highlighted. The genetic sex of the embryo is determined at fertilization by the kind of sperm that fertilizes the secondary oocyte (thus XY embryo is a genetic male and XX embryo is genetically female). However, the development of the male and female morphologic characteristics do not begin until the 7th week. The genital systems in both the male and female at the very early stage of embryonic development are similar, thus this initial period of genital development is referred to as the indifferent stage of sexual development.
This indifferent stage of gonadal (developing testes and/or ovaries) development starts during the 5th week. The gonads (developing testes, in this case) are derived from three sources:
- Mesothelium lining the posterior abdominal wall
- The underlying mesenchyme (embryonic connective tissue)
- Primordial germ cells
Prior to the 7th week, the gonads are referred to as indifferent gonads. Proliferation of the three sources mentioned above produce a bulge on the medial side of the mesonephros (kidney which appears late in the 4th week and functions for about 4 weeks before a permanent kidney develops). That bulge is the gonadal ridge – the primordium of the testes in males. With development, the gonadal ridge gives rise to the gonadal cords which grow into the underlying mesenchyme (embryonic connective tissue), and finally, under the influence of the Sox9 and Fgf9 genes, the gonadal cords differentiate into the seminiferous cords (primordia of the seminiferous tubules). The Sox9 gene is essential for testicular determination. However, the SRY gene on the short arm of the Y chromosome acts as a switch that directs development of the indifferent gonad into a testis. The gonadal cords are induced to condense and form the rete testis.
Shortly after, the dense tunica albuginea is formed when the seminiferous cords lost their connection with the surface epithelium. The rete testis becomes continuous with 15 to 20 mesonephric tubules that become efferent ductules (Latin: ductuli efferentes). These ductules are connected with the mesonephric duct, which becomes the duct of the epididymis. Under the influence of testosterone produced by the fetal testes in the 8th week, the proximal part of each mesonephric ducts becomes highly convoluted to form the epididymis.
The caudal end of each mesonephric duct gives rise to lateral outgrowths. Those outgrowths from the seminal glands. The part of the mesonephric duct between the duct of seminal glands and the urethra, becomes the ejaculatory duct.
During embryonic development, the testes are situated at the posterior abdominal wall. However by the 26 weeks, the testes have descended, retroperitoneally through the inguinal canal into the scrotum to assume their permanent location. This descent of the testes is controlled and induced by several factors including enlargement of the processus vaginalis which guide the testes through the inguinal canal.
Urethra & Glands
The urethra develops mainly from the splanchnic mesenchyme, while its epithelium is derived from the endoderm of the urogenital sinus. The prostate is derived from the prostatic part of the urethra and grows into the surrounding mesenchyme. With further development, the glandular epithelium of the prostate is formed and it differentiates from the associated mesenchyme into the dense stroma and smooth muscle of the prostate. Finally the bulbourethral glands, pea-sized structures, develop from paired outgrowths from the spongy part of the urethra.
Development of the male external genital organs is mainly induced by testosterone produced by cells of the fetal testes. The penis is formed by elongation of the phallus. Urethral folds form the lateral wall of the urethral groove on the ventral surface of the penis. Fusion of the urethral folds along the ventral surface of the penis forms the spongy urethra. During the 12th week, a circular ingrowth of ectoderm occurs at the periphery of the distal end of the penis – the glans penis. That ingrowth breaks down to form the prepuce (foreskin) – a covering fold of skin. The scrotum is formed by fusion of the labioscrotal swellings which grow towards each other. The line of fusion of these folds (labioscrotal swellings) is clearly visible as the scrotal raphe.
Gross And Microscopic Structures
The following are the histological and gross anatomy of the male reproductive organs:
External Genital Organs
This is the male copulatory organ. It has a long shaft or body and an enlarged bulbous-shaped tip called the glans penis, which is prolonged by a foreskin called the prepuce in uncircumcised males. The penis consists of an attached root (radix) (consisting of the bulb of the penis with a crus on each side) in the perineum and a free, pendulous body (corpus or shaft) which is completely covered with skin. The penis is the organ by which the sperm is introduced into the female reproductive system. It contains spongy tissues that becomes turgid and erect when filled with blood. Erectile tissues of the penis include the corpus spongiosum and corpora cavernosa.
The corpora cavernosa form most of the shaft of the penis. Those structures are enveloped by tunica albuginea (deep and superficial layers), and contains most of the blood (due to its rich vasculature) in the penis during penile erection. The corpus spongiosum is also covered by tunica albuginea, and is continuous with the other corpora – the cavernosa. The corpus spongiosum is traversed by the urethra. In addition, it adjoins the median groove on the urethral surface of the conjoined corpora cavernosa. It is cylindrical and tapers slightly and extends into the glans penis. As a mass of spongy tissue (or erectile tissue), the corpus spongiosum also allows rush of blood into itself during erection.
Other structures of the penis include the superficial penile fascia, suspensory ligament of penis and vasculature. The superficial penile fascia is devoided of fat, and consists of loose connective tissue invaded by fibres of the dartos muscle from the scrotum. The suspensory ligament of penis are the fundiform and triangular ligaments, and they both support the body of the penis. Those ligaments are made up of elastic fibres and they are continuous with the penile fascia.
The scrotum is the pouch-like structure formed from the lower part of the abdominal wall and hangs behind the penis. It houses and protects the testes and spermatic cords, and keeps the testes at a temperature of about 2-4°C lower than the body temperature.
The scrotal subcutaneous tissue has no fat, but contains a part of the panniculus carnosus, the dartos muscle, which sends a sheet into the midline fibrous septum of the scrotum. During times of lower temperature, the cremaster muscle contracts to pull the scrotum closer to the body for a higher temperature, while the dartos muscle gives the scrotum its wrinkled appearance. At an increased temperature, the dartos and cremasteric muscles relax to bring down the scrotum away from the body, thus removing the wrinkled appearance. The scrotal wall is a thin layer of skin lined with smooth muscle tissue (dartos fascia or Colles’ fascia). The skin overlying the scrotum contains pigment and has many sebaceous (oil-producing) glands, sweet glands and some hair. Externally, the right and left components of the scrotum are separated by a midline ridge called the raphe.
Internal Genital Organs
The testes (plural for testis) are the two oval-shaped male internal genital organs that produce sperm and the hormone testosterone, and lies within the scrotum. The left testis lies slightly lower than the right.
Each testis is made of tightly coiled structures called seminiferous tubules, to which, there are adjacent cells called Leydig cells or interstitial cells of Leydig and these interstitial cells produce testosterone in the presence of luteinizing hormone (LH). The seminiferous tubules open into a series of channels called the rete testis. Sperm move from the rete testis to the epididymis. The epididymis is attached to the posterolateral surface of the testis. A direct continuation of the epididymis is another structure called the vas deferens (ductus deferens). Covering the testis is a thick fibrous tissue, the tunica albuginea. The ventral and lateral surfaces of the testis lie free in a serous space formed by the overlying tunica vaginalis, a remnant of the fetal processus vaginalis.
The epididymis (singular for epididymides) is a firm single tube of about 7 meters long, highly coiled and packed together by fibrous tissue. It is attached behind the testis, with the ductus deferens to its medial side. The epididymis acts as a site for maturation and storage of spermatozoa. Lying between the epididymis and the testis is a narrow slit, the sinus of the epididymis. This sinus lies within the tunica vaginalis. The epididymis is divided into three parts. The three parts are the head, body, and tail. The vas deferens is continuous with the epididymis at the tail.
Vas (Ductus) Deferens
The ductus deferens (Latin: ductus deferentes) is the continuation of the duct of the epididymis. The vas deferens has a relatively thick muscular wall and a minute lumen. It begins at the tail of the epididymis, at the level of the inferior pole of the testis and ascends posterior to the testis, medial to the epididymis. It forms the primary component of the spermatic cord. The vas deferens runs further upwards and enlarges to form the ampulla of ductus deferens before terminating by joining the duct of the seminal gland to form the ejaculatory duct.
Seminal Glands (Seminal Vesicles)
The seminal vesicle is an elongated structure aproximately 5cm long, and lies between the fundus of the bladder and the rectum. The seminal glands are obliquely placed superior to the prostate and do not store sperm, but rather, they secrete a thick alkaline fluid with fructose (an energy source for sperm), and a coagulating agent that mixes with the sperms as they pass into the ejaculatory ducts and urethra. The superior ends of the seminal vesicles are covered with peritoneum. The duct of the seminal gland joins the ductus deferens to form the ejaculatory duct.
These structures are slender tubes that arise by the union of the ducts of the vas deferens and seminal vesicle. It is about 2.5cm long and arises near the neck of the bladder. The ejaculatory ducts run to pierce the prostate and empty into the urethra that runs through the prostate gland.
Prostate (Prostate Gland)
The prostate gland is about 3cm long, 4cm wide and 2cm in anteroposterior (AP) depth, and it is the largest accessory gland of the male reproductive system.
The prostate can be divided in two ways: by zone, or by lobe.
- Peripheral zone (PZ) - It is from this portion of the gland that ~70–80% of prostatic cancers originate.
- Central zone (CZ) - This zone surrounds the ejaculatory ducts. The central zone accounts for roughly 2.5% of prostate cancers although these cancers tend to be more aggressive and more likely to invade the seminal vesicles.
- Transition zone (TZ) - 10–20% of prostate cancers originate in this zone. The transition zone surrounds the proximal urethra and is the region of the prostate gland that grows throughout life and is responsible for the disease of benign prostatic enlargement.
- Anterior fibro-muscular zone (or stroma) - This zone is usually devoid of glandular components, and composed only, as its name suggests, of muscle and fibrous tissue.
- Anterior lobe (or isthmus)-roughly corresponds to part of transitional zone
- Posterior lobe-roughly corresponds to peripheral zone
- Lateral lobes-spans all zones
- Median lobe (or middle lobe)-roughly corresponds to part of central zone
The prostate is surrounded by a capsule, the fibrous capsule of the prostate, which contains the blood vessels and nerves of the prostate. This fibrous capsule is, in turn, surrounded by the visceral layer of the pelvic fascia, forming a fibrous prostatic sheath. The prostate mainly functions to secrete a milky fluid that adds about 20% to the volume of semen. This milky fluid also plays a role in the activation of spermatozoa. The prostatic ducts, which are about 20-30 in number, open mainly into the prostatic sinuses that lie on either sides of the seminal colliculus on the posterior wall of the prostatic urethra.
These are the third accessory glands of the male reproductive system, and they are also called the Cowper glands. They are small round structures about 1cm long, that lie posterolateral to the intermediate part of the urethra. They empty directly into the urethra and secrete slippery fluids that lubricate the glans penis when the penis is erect.
Blood Supply And Venous Drainage
The penis receives three pairs of arteries which are all branches of the internal pudendal artery. These arteries are:
- artery to the bulb
- deep artery of the penis
- dorsal artery of the penis
The penis is drained by:
- accompanying veins of the arteries mentioned above, but mainly by the deep dorsal vein. All the veins return blood to the internal pudendal veins.
- superficial and deep external pudendal arteries
- branches from the internal pudendal arteries
Venous drainage includes:
- external pudendal veins (superficial and deep to the great saphenous vein)
- testicular arteries, which run down from the aorta
The testes are drained by the:
- pampiniform plexus
- branches of the testicular artery
- Venous return is by the pampiniform plexus
- branch of the superior or inferior vesical artery ( a very tiny artery)
- Venous drainage is by the pampiniform plexus
The seminal vesicles are supplied by:
- branches from the inferior vesicle and middle rectal arteries
These glands are drained by:
- inferior vesicle and middle rectal veins, and these veins accompany the arteries (vena comitantes)
- branches of the superior and/or inferior vesical arteries
They are drained by the vena comitans that join into the prostatic and vesical venous plexuses.
- branches of the internal iliac artery including the inferior vesical arteries, internal pudendal and middle rectal arteries
The prostate is drained by:
- the prostatic venous plexus, this in turn drains into the internal iliac veins
- supplied by one of the three branches of the common penile artery - the bulbo-urethral artery
- Pudendal nerves
- Perineal nerves
- Posterior scrotal and dorsal nerves
- Pelvic splanchnic nerves ( S2 and S3 nerves)
- Ilioinguinal nerve (L1)
- Scrotal branches of the perineal nerve (S3)
- Posterior femoral cutaneous nerve (S2)
- Sympathetic nerves
- Sympathetic nerves from the coeliac ganglion
Ductus Deferens and Seminal Vesicle
- Pelvic plexus
- Pelvic splanchnic nerves